How can multiple people work on the same file?
Sadly there is no built in way to do this. So far the popular method has been to use git or svn which, while not built for this, can be made to work. (although keeping track of who uses what file is a complicated, I personally use a google sheet, but people forget to update it)

The down side with git and svn is that they can require some technical knowledge, such as command line usage. There are GUI apps (Github Desktop,Source Tree) that help; however I always find myself having to resort to using the command line.